UL 508A section 35.2.2.1 dual-voltage transformer

[TonyTurney]

In the UL508A standard for industrial control panels, section 35.2.2.1 states: “35.2.2.1 A set of branch circuit fuses or an inverse-time circuit breaker provided for both the primary and secondary sides of a power transformer shall be sized in accordance with Table 35.2. A transformer with multiple secondary windings shall be provided with a set of branch circuit fuses or an inverse-time circuit breaker for each secondary sized in accordance with Table 35.2” [emphasis added].

Does this require that each winding of a dual-voltage transformer have separate protection? Or is a parallel connection of two windings for the lower voltage still considered one secondary, and "each secondary" relates to something other than simply interconnecting the windings of a dual voltage 120/240 transformer, for instance?

The statement in the code refers to “branch circuit fuses” – that would seem to put the emphasis on protecting the conductors, rather than the windings, with both windings considered part of the same circuit.

In the diagram below, a 480/240 - 240/120 transformer is shown with a 480 volt primary supply and a 120 volt secondary connection. In the upper circuit, fuse F-040127A is used after the parallel connection of both windings, while in the lower circuit fuses F-040133A and F-040135 protect each winding separately before the parallel connection. I have never seen this connection specified.




Also, if the intent is to protect the transformer windings, wouldn't the same apply to a dual-voltage primary, as well, say, if I used the same 480/240 - 240/120 transformer with a 240 volt primary supply?

Thanks,

Tony

 

[LarryFine]

To me, combined secondaries, whether in series or parallel, become a single secondary for this discussion.

I think they're talking about multiple secondaries used as separate sources for separate, isolated circuits.

 

[Jraef]

The lower drawing is not really something that people do very often, but people DO use the secondary windings separately as separate isolated circuits quite often. Each individual winding would have half of the rated capacity of the transformer, so they need separate fusing.

 

[TonyTurney]

RE: UL 508A section 35.2.2.1 dual-voltage transformer

To me, combined secondaries, whether in series or parallel, become a single secondary for this discussion.

I think they're talking about multiple secondaries used as separate sources for separate, isolated circuits.

Larry,

Thanks for the reply. I agree with you that the intent seems to be to protect the branch circuit supplied by each source.

Our practice has been to treat each derived supply independently. Wiring the secondary windings in parallel makes one 120 volt supply, so we use only one branch circuit fuse in such a case. Wiring them to make a 120/240 supply requires two branch circuit fuses.

I was concerned, though, about the phrase "multiple secondary windings" with no further qualification as to the usage of those windings, as well as the distinction made elsewhere between single-phase secondaries with only 2 wires versus secondaries with more than 2 wires (specifically relating to primary-only versus primary and secondary protection in 35.2.1.2), and wanted to make sure to apply the code accurately. It just doesn't seem to be clearly stated, although I think I know what is intended.

Thanks again,

Tony

 

[TonyTurney]

RE: UL 508A section 35.2.2.1 dual-voltage transformer

The lower drawing is not really something that people do very often, but people DO use the secondary windings separately as separate isolated circuits quite often. Each individual winding would have half of the rated capacity of the transformer, so they need separate fusing.

I agree - see my reply to LarryFine. I think the description of the protection as "branch circuit" is the key.

Thanks

 

[synchro]

I agree with the distinction being made above relating to the mention of a branch circuit.

I don't have the UL standard itself, but in the following slide about control transformers under UL508A the protection requirements presented relate to the external load(s) that are applied. Or if the secondary winding has a connection to outside the panel (because I suspect it's presumably more vulnerable to a fault condition or unanticipated extra loading).
So in this context the requirement for separate protection of each secondary winding or tap I think is relatively obvious because each connected load can (and in general will) be different. But with two paralleled matched secondaries the load on each winding will be essentially equal, and a single fuse no larger than the combined rating is appropriate.

Also, the having a separate fuse for each winding before paralleling has some undesired characteristics. For example, if one fuse blows due to an overload then the other one (which now has to support twice the current) certainly will also.
And so the extra cost provides little or no benefit.

https://www.industry.usa.siemens.co...FPA control panel_part2_April 2014.pdf#page=8